The invention relates to an apparatus for forming the edge of a container of synthetic thermoplastic material which is open at the top and ends in an outer upwardly arched and than downwardly extending flange.
Machines for edging containers of synthetic thermoplastic material which are open at the top and terminate in an outer flange based on the deformation of the said flange which is heated to a temperature such as to render the material plastic by means of several endless screws with threads of variable cross-section are known from British Patent Nos. 951,242 and 1,101,031. By these means, the flange is deformed and rounded into the desired shape by a plurality of edging members which rotate relative to the container which is to be edged.
In these machines heating of the flange is an integral part of the edging cycle, and in the event of the edging members being stopped a plurality of containers remain held between the channels formed by the heating rollers, which because of their high temperature rapidly cause melting of the flanges of the trapped containers, resulting in the presence of molten material between the threads of the said rollers.
In this situation it is necessary to stop the machine for a long period so that the trapped containers can be removed and the threaded rollers can be cleaned.
A further loss of time is involved in returning the threaded rollers to their proper temperature before edging is resumed.
In the edging machines mentioned above, the band which bends and rounds the flange cannot perform its proper function when the thickness of the flange exceeds specific values.
In band edgers, each edging member compresses the flange radially at the points of contact between the flange and the edging rollers, making it difficult to maintain the diameter of the container mouth perfectly circular and the thickness of the edge obtained constant.
With the edging machines mentioned, containers, for example of crystalline polystyrene, do not withstand the torsion due to rotation of the container with respect to the band. This torsion sets up tensions which result in collapse of the material in question. With containers of polypropylene for example, the edging permits easy unrolling of the edge because the latter is not sufficiently stabilised on leaving the machine.